The frequency band available around 60 GHz offers up to 9 GHz of bandwidth for use in high-speed wireless communication and negligible interference. 60 GHz band, in comparison with the current low radio-frequency (RF) wireless services, is found as a very promising candidate for very high throughput not only for wireless personal area networks (WPANs), but also for local area networks (LAN). Several industrial and standardization efforts have been carried out, such as IEEE 802.15.3c, ECMA and Wireless-HD, to promote the global use of multi-gigabit 60-GHz wireless technology. Wireless transceivers operating in this frequency range often utilize phased arrays (often referred as beamformers) to relax the wireless link budget. As a result, beamforming circuits are needed in such systems. Beamforming requires two operations: in a transmitter, splitting the signal over the different antenna paths followed by phase shifting the signals, and, in a receiver, phase shifting the signals in the different antenna paths and then combining the signals. The beamformer can be implemented in various ways, e.g. in the signal path at radio frequency (RF), in the local oscillator (LO) path or at analogue baseband (BB). Among these possibilities, analogue baseband beamforming features high robustness, as operations are performed at much lower frequencies than in the other approaches (i.e. at baseband frequency), yielding a low sensitivity to parasitic elements. Another important function in wireless receivers is low-pass filtering required to suppress interferences outside the wanted channel and to avoid aliasing by the sampling action of the analogue-to-digital converter.
A block scheme of a conventional wireless receiver utilizing an analogue baseband beamforming and low-pass filtering is shown in FIG. 1. In the different RF front-ends (RF1, RFN) of the receiver the input signal is amplified, downconverted to baseband frequency and then phase-shifted. The signals from the different antenna paths are then combined in the summator (COM), filtered in the low-pass filter (LPF) and further amplified in a variable gain amplifier (VGA).
Patent application EP 2267919 proposes system-on-chip realization of a wireless communication receiver using beamforming shown schematically in FIG. 2, where the RF front-ends are spread across the chip as they are driven by separate antennas and thus long on-chip interconnects are required to drive the signal to the summator (often referred as a combiner) where they are combined. Their inevitable length results in high parasitic capacitance (Cp) and greatly limits the signal bandwidth. Additional buffers/amplifiers to compensate for the signal losses are used which however increase the power consumption and degrade the spurious free dynamic range of the receiver. Further, due to stringent requirements of the low-pass filter on noise, cut-off frequency and transfer function shape, the use of large capacitors in the low-pass filter is required which further increases the overall chip area and power consumption.
In the paper “A 900-MHz bandwidth analog baseband circuit with 1-dB step and 30-dB gain dynamic range” (M. Hosoya et al., Proc. 2010 IEEE European Solid-State Circuits Conference, pp. 466-469) an analog baseband section is presented. It implements low-pass filtering, variable gain amplification with a DC offset compensation path. The low-pass filtering is implemented using a classic gm-C topology. The system presented in the paper does not implement any phase shifting. In the paper a single antenna path baseband section is implemented and no issues related to phased array implementation are addressed, e.g. the long interconnects with a parasitic capacitance and combination of the signal of antenna paths placed quite far from each other.
Hence, there is a need for a solution where long on-chip interconnects (and consequently signal degradation) are avoided. In such solution there would be no more need for buffering to compensate for parasitic capacitance. There is a need for a solution that facilitates the implementation of a baseband section for phased arrays.